Spectrometers, colorimeters, and other optical instruments have been used for years to measure various properties of materials (e.g., hue, lightness, chemical composition, etc.) by illuminating a material sample and analyzing light or other radiation that is either reflected by or transmitted through the sample. Due to the large range of perceivable differences in reflected light, it is desirable for these instruments to have a high degree of accuracy, repeatability, and inter-instrument agreement.
Some existing methods for manufacturing optical instruments meet these goals by driving the hardware output of the optical instruments toward an accepted standard. For example, instruments may be constructed with tight-tolerance components and then mechanically tuned and adjusted to the accepted standard during the manufacturing process. These methods, however, do not adequately account for changes in the instrument in the field due to temperature, age, environmental conditions, etc. This is left to simple calibration procedures, which are often inadequate. Also, these methods are limited in the types of components that they can use. For example, low cost, efficient illumination sources, such as light emitting diodes (LED's) cannot be easily used because they are not currently available with sufficiently tight tolerances, and because their spectral output varies with temperature.
Other existing methods for manufacturing optical instruments attempt to use looser tolerance components, such as LED's, by developing instrument-level correction factors that are applied to the hardware output in an attempt to bring it into conformance with the accepted standard. The correction factors are developed based on an extensive and often expensive, characterization of the instrument as a whole. Instrument level characterizations, though, are often not adequate to compensate for complex non-linear changes in the instruments due to changes in temperature and other environmental changes that affect the individual instrument components (such as LED's) in the field.
Still other attempts have been made to address the shortcomings of LED's, however, these also leave room for improvement. For example, various methods have been developed that attempt to stabilize the output of an LED by manipulating its current and voltage drop. Also, some known methods involve heating an LED in an attempt to make its output constant. All of these methods, however, add additional cost and complexity to optical instruments, and still fail to give the optical instrument a desired level of accuracy.